Many patients have visual errors associated with the refractive properties of the eye, such as nearsightedness, farsightedness, and astigmatism. Nearsightedness occurs when light focuses in front of the retina, while farsightedness occurs when light refracts to a focus behind the retina. Astigmatism may occur when the corneal curvature is unequal in two or more directions.
There are numerous prior surgical approaches for reshaping the cornea. Over the years, surgical laser systems have replaced manual surgical tools in ophthalmic procedures. For instance, in the well-known procedure known as LASIK (laser-assisted in situ keratomileusis), a laser eye surgery system providing a near-infrared femtosecond laser is used to cut a flap in the cornea, and another laser system providing ultraviolet radiation is used for ablating and reshaping the anterior surface of the cornea to correct nearsightedness or farsightedness. Other surgical approaches for reshaping the cornea include all laser LASIK, femto LASIK, corneaplasty, astigmatic keratotomy, corneal relaxing incision (hereinafter “CM”), Limbal Relaxing incision (hereinafter “LRI”), photorefractive keratectomy (hereinafter “PRK”) and Small Incision Lens Extraction (hereinafter “SMILE”).
Incisions such as Astigmatic Keratotomy, Corneal Relaxing Incision (CRI), and Limbal Relaxing Incision (LRI), are made at a depth in the cornea in a well-defined manner so as to enable the cornea to become more spherical. Arcuate incisions are conical incisions made in the cornea. Typically, to prevent an incision from penetrating entirely through the cornea, an arcuate incision is made that does not penetrate the posterior surface of the cornea. Some laser eye surgery systems are capable of making intrastromal arcuate incisions using a laser so that the incision is completely contained within the thickness of the cornea, and does not penetrate either the anterior or posterior surfaces of the cornea.
For a given astigmatic distortion, nomograms tables) are often consulted to prescribe the depth of the arcuate incision and the angle appropriate to correct astigmatism. These tables predict the curvature correction of the cornea as a function of the incision depth and position.
The mechanical properties of the cornea, however, vary from person to person. While there are available nomograms that attempt to account for these differences by including parameters such as age, sex, and intraocular pressure to provide a better estimate in treatment planning, these tables are constructed on a trial-and-error basis from observational evidence from refractive surgeries. Although more popular nomograms, such as the Abbott LRI calculator, are available, many doctors often generate their own nomograms based on their own surgical experience. Hence, there is no consensus on a benchmark nomogram. Furthermore, current astigmatism treatment is performed as an open loop process in that the treatment is not adjusted in response to changes in the curvature of the cornea. Thus, current treatment planning methods for astigmatism are simple and provide sub-optimal results.
Therefore, there is a need for improved surgical laser apparatus and methods of treatment planning for treating astigmatism of the eye.